The present invention relates in general to activated carbon materials, and more specifically to carbon adsorbents suitable for use as storage media for light fuel gases.
Porous carbons and carbon molecular sieving materials are widely used in adsorption applications which include gas separations and other chemical applications based on physical adsorption. The following U.S. patents are typical of the prior art and teach a wide variety of materials and processes relating to the current applications for activated carbons:
U.S. Pat. No. 4,205,055 --Maire et al. PA1 U.S. Pat. No. 4,261,709 --ltoga et al. PA1 U.S. Pat. No. 4,263,268 --Knox et al. PA1 U.S. Pat. No. 4,526,887 --Sutt, Jr. PA1 U.S. Pat. No. 4,540,678 --Sutt, Jr. PA1 U.S. Pat. No. 4,594,163 --Sutt, Jr. PA1 U.S. Pat. No. 4,775,655 --Edwards et al. PA1 U.S. Pat. No. 4,832,881 --Arnold, Jr. et al. PA1 U.S. Pat. No. 4,902,312 --Chang PA1 U.S. Pat. No. 5,071,450 --Cabrera et al. PA1 U.S. Pat. No. 5,086,033 --Armor et al. PA1 U.S. Pat. No. 5,098,880 --Gaffney et al. PA1 U.S. Pat. No. 5,208,003 --Simandl et al. PA1 U.S. Pat. No. 5,232,772 --Kong PA1 U.S. Pat. No. 5,298,313 --Noland PA1 U.S. Pat. No. 5,300,272 --Simandl et al.
Although the above prior art teaches porous carbons for a wide variety of usage, the above patents do not teach the use of these materials as a storage medium for light fuel gases at the supercritical conditions required for such applications. Furthermore, the above prior art requires that the activated carbon be formed by multiple process steps which are both time consuming and costly, and do not provide for a carefully controlled pore size range which is a requirement for optimal gas storage.
U.S. Pat. Nos. 4,839,331 and 4,040,990 are directed to the formation of carbonaceous adsorbents from pyrolized polysulfonated polymers, but do not teach or suggest the use of these materials for gas storage. The '331 and '990 patents include the use of starting materials which are macroporous, and require multiple steps in order to achieve the activated carbon product. Furthermore, the patents teach the formation of activated carbons having a multimodal pore size distribution with a pore sizes ranging between 50-10,000 .ANG. . Activated carbons having pore sizes with such a wide size distribution would not be suitable for use as gas storage materials.
U.S. Pat. Nos. 4,716,736 and 5,385,876 to Schwarz et al. teach the use of activated carbon materials suitable for use as an adsorbent for light gases such as hydrogen and methane. These patents however require methods of preparation in which the activated carbon is formed by multiple process steps.
In addition, an article entitled Influence of Pore Geometry on the Design of Microporous Materials for Methane Storage, by R. Cracknell, P. Gordon and K. E. Gubbins, which appears in J. Phys. Chem., 1993, 97, 494-499 addresses the advantage of storing methane by adsorption in microporous materials, and the merits of currently available zeolites and porous carbons. The article is theoretical in nature, and concludes that the prior art fails to teach or provide the technology to economically store methane, and that considering the state of the art, that it would be more economical to store methane as a bulk fluid.
The article observes that key factors which are important in the design of a suitable microporous material are first that the microporous material be such that the amount adsorbed minus the amount retained, when the methane is released, should be a maximum. Second, that the microporosity (fraction of the micropore volume) should be a maximum; that is the space taken by the atoms of the microporous material and the space wasted by poor packing of the crystallites should both be minimized. The authors found that adsorption in a porous material offers the possibility of storing methane at high density while maintaining moderate physical conditions for the bulk phase, and that the search for a suitable material is currently an active area of research.
It can therefore be seen front above that there is a continuing search for suitable light fuel gas storage materials, and that a key objective in developing such a material is the formation of a geometry that provides for optimum pore size distribution that maximizes the excess adsorption, i.e. the density in the pore minus the bulk density, for a given temperature and pressure. The key objective in developing such a material, is to provide a geometry which will provide the maximum storage which is recoverable for use.